Going underground in search of dark matter strikes

THE chill hits me as soon as the door shuts. Then the floor starts to rumble. The lift I've just entered feels like an original from the late 1800s, when miners made the same trip in search of iron ore. But my guide and I (pictured above) are seeking a much more elusive substance: dark matter.

The same stuff was in the spotlight last week due to a tentative sighting in space, but we head in the opposite direction and descend into the Soudan Mine in northern Minnesota, home to rival experiments CDMS and CoGeNT.

Like their space-based counterpart, AMS, both are designed to detect weakly interacting massive particles, the favoured form of dark matter. But while AMS looks for positrons given off when WIMPs collide, the hope down here is that a WIMP will knock into a heavy atom in supercooled crystals, making the atom's nucleus recoil and sending out a signal. Hundreds of metres of rock above us protect the detectors from atmospheric particles that could mimic such a strike.

My guide, Jeter Hall, works on the CDMS detector, which he shows me first. About eight layers of shielding surround the array of supercooled silicon and germanium crystals – including a layer of lead salvaged from shipwrecks – so the experiment takes up most of the room. CoGeNT's single, 440-gram germanium crystal sits in a room next door, in a smaller box covered with polyethylene and lead. In 2011, this detector caught whiffs of a WIMP weighing 7 gigaelectronvolts (GeV), but the signal was inconclusive.

CDMS meanwhile saw two potential "events" over its last run in 2008 but these weren't sure-things. And, because it only looks for WIMPs weighing 100 GeV or more, it couldn't verify CoGeNT's result. CDMS has since been upgraded so it is sensitive below 10 GeV: results are due later this year.

Lack of sensitivity in these detectors may explain why WIMPs have evaded direct detection so far. In one new model of dark matter, most WIMPs in our galaxy live in a diffuse spherical cloud, while about 15 per cent have been drawn into a disc, like a shadow Milky Way (see "Twist in dark matter tale hints at shadow Milky Way"). WIMPs in this dense disc would be more likely to hit a detector but as they are keeping pace with Earth in its flight around the galaxy, they would collide with less energy than expected.

If WIMPs keep failing to show up, could dark matter be something entirely different? "It can keep you up at night that the thing you're searching for might not be there at all," says Hall. "But it's compelling enough for me to vote with my feet and work on it anyway."

This article appeared in print under the headline "Underground lab seeks dark particle strikes"

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